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PubMedDate not supplied

Quantitative level of protection offered to workers by ACGIH threshold limit values occupational exposure limits.

The details of the example or modeling methodologies used herein are not critical to the general point of this article, which advises the estimation of residual risk at the OEL by using some quantitative modeling structure. Specifically, the authors believe that an explicit attempt to gauge the level of residual risk at the OEL based on conceptual stochastic models with transparent and testable assumptions could be seen as an important enhancement to the process. This is especially true in sharing the OEL deliberations and explaining OEL decisions to the stakeholders. Indeed, if this approach is used, it is critically important to understand and continually communicate that this "cloud of uncertainty" represents model estimates in which the true risk would most likely be less than worst case estimates and could possibly be zero. It is also possible but highly unlikely that it could be higher than the worst case upper-bound estimate. The above quantitative estimation scheme represents a possible improvement that could provide a reasoned attempt on the part of the risk assessors to use rational science (i.e., conceptual models with transparent and testable assumptions) to inform all of the OEL users and stakeholders of their meaning.

Air Pollutants, Occupational↗

PubMed1993

Uncertainty in cancer risk estimates.

Several existing databases compiled by Gold et al. for carcinogenesis bioassays are examined to obtain estimates of the reproducibility of cancer rates across experiments, strains, and rodent species. A measure of carcinogenic potency is given by the TD50 (daily dose that causes a tumor type in 50% of the exposed animals that otherwise would not develop the tumor in a standard lifetime). The lognormal distribution can be used to model the uncertainty of the estimates of potency (TD50) and the ratio of TD50's between two species. For near-replicate bioassays, approximately 95% of the TD50's are estimated to be within a factor of 4 of the mean. Between strains, about 95% of the TD50's are estimated to be within a factor of 11 of their mean, and the pure genetic component of variability is accounted for by a factor of 6.8. Between rats and mice, about 95% of the TD50's are estimated to be within a factor of 32 of the mean, while between humans and experimental animals the factor is 110 for 20 chemicals reported by Allen et al. The common practice of basing cancer risk estimates on the most sensitive rodent species-strain-sex and using interspecies dose scaling based on body surface area appears to overestimate cancer rates for these 20 human carcinogens by about one order of magnitude on the average.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1990

Estimation of radionuclide ingestion: the "PATHWAY" food-chain model.

This paper describes the structure of the dynamic food-chain model PATHWAY and its utility for estimating radionuclide ingestion after fallout deposition from nuclear testing in Nevada. Model input requirements are described and output examples are provided. The basic output of PATHWAY is the time-integrated radionuclide ingestion by humans per unit fallout deposition (Bq per Bq m-2). Output specific to sex, age, life-style (diet), location (agricultural practice), event (calendar date), and radionuclide may be generated. Uncertainties of model predictions, based on "Monte Carlo" simulations using parameter value distributions, are described. Results of a sensitivity analysis, based on a ranking of partial correlation coefficients, are reviewed to illustrate the relative importance of parameters and associated transport pathways. Output data for 131I and 137Cs in milk are compared with predictions from several well known food-chain models. Preliminary efforts to validate PATHWAY results with real data sets are described.

Adolescent↗

PubMed1997

Adaptation of ECOSYS-87 to Hong Kong environmental conditions.

This paper describes the adaptation work carried out on the radioecological model ECOSYS for radionuclide transfer in the Hong Kong ecological environment. The adapted model predicts that the ingestion dose due to dry deposition in Hong Kong shows less pronounced seasonal dependence than that in Germany. This is mainly attributed to differences in climate, agricultural and farming practices adopted in the two places. Brief discussions on model sensitivity, uncertainty, and validation are also given.

Agriculture↗

PubMed2004

A model to predict bladder shapes from changes in bladder and rectal filling.

The purpose of this study is to develop a model that quantifies in three dimensions changes in bladder shape due to changes in bladder and/or rectal volume. The new technique enables us to predict changes in bladder shape over a short period of time, based on known urinary inflow. Shortly prior to the treatment, the patient will be scanned using a cone beam CT scanner (x-ray volume imager) that is integrated with the linear accelerator. After (automated) delineation of the bladder, the model will be used to predict the short-term shape changes of the bladder for the time interval between image acquisition and dose delivery. The model was developed using multiple daily CT scans of the pelvic area of 19 patients. For each patient, the rigid bony structure in follow-up scans was matched to that of the planning CT scan, and the outer bladder and rectal wall were delineated. Each bladder wall was subdivided in 2500 domains. A fixed reference point inside the bladder was used to calculate for each bladder structure a "Mercator-like" 2D scalar map (similar to a height map of the globe), containing the distances from this reference point to each domain on the bladder wall. Subsequently, for all bladder shapes of a patient and for all domains on the wall individually, the distance to the reference point was fitted by a linear function of both bladder and rectal volume. The model uses an existing bladder structure to create a new structure via expansion (or contraction), until the expressed volume is reached. To evaluate the predictive power of the model, the jack-knife method was used. The errors in the fitting procedure depended on the part of the bladder and range from 0 to 0.5 cm (0.2 cm on average). It was found that a volume increase of 150 cc can lead to a displacement up to about 2.5 cm of the cranial part of the bladder. With the model, the uncertainty in the position of the bladder wall can be reduced down to a maximum value of about 0.5 cm in case the bladder volume increase is known. Furthermore, it was found that a change in rectal filling causes a shift of the bladder, while its shape is hardly influenced. In conclusion, we developed a model that describes the bladder shape and position as a function of the bladder volume and the rectal filling. The model accurately describes the complex shape of the bladder as it works on each domain of the bladder separately.

Biophysical Phenomena↗

PubMed2002

Emissions from ships in the northwestern United States.

Recent inventory efforts have focused on developing nonroad inventories for emissions modeling and policy insights. Characterizing these inventories geographically and explicitly treating the uncertaintiesthat result from limited emissions testing, incomplete activity and usage data, and other important input parameters currently pose the largest methodological challenges. This paper presents a commercial marine vessel (CMV) emissions inventory for Washington and Oregon using detailed statistics regarding fuel consumption, vessel movements, and cargo volumes for the Columbia and Snake River systems. The inventory estimates emissions for oxides of nitrogen (NOx), particulate matter (PM), and oxides of sulfur (SOx). This analysis estimates that annual NOx emissions from marine transportation in the Columbia and Snake River systems in Washington and Oregon equal 6900 t of NOx (as NO2) per year, 2.6 times greater than previous NO, inventories for this region. Statewide CMV NO, emissions are estimated to be 9,800 t of NOx per year. By relying on a "bottom-up" fuel consumption model that includes vessel characteristics and transit information, the river system inventory may be more accurate than previous estimates. This inventory provides modelers with bounded parametric inputs for sensitivity analysis in pollution modeling. The ability to parametrically model the uncertainty in commercial marine vessel inventories also will help policy-makers determine whether better policy decisions can be enabled through further vessel testing and improved inventory resolution.

Air Pollutants↗

PubMed1998

Signal detection by human observers: a cutoff reinforcement learning model of categorization decisions under uncertainty.

Previous experimental examinations of binary categorization decisions have documented robust behavioral regularities that cannot be predicted by signal detection theory (D.M. Green & J.A. Swets, 1966/1988). The present article reviews the known regularities and demonstrates that they can be accounted for by a minimal modification of signal detection theory: the replacement of the "ideal observer" cutoff placement rule with a cutoff reinforcement learning rule. This modification is derived from a cognitive game theoretic analysis (A.E. Roth & I. Erev, 1995). The modified model reproduces all 19 experimental regularities that have been considered. In all cases,it outperforms the original explanations. Some of these previous explanations are based on important concepts such as conservatism, probability matching, and "the gambler's fallacy" that receive new meanings given the current results. Implications for decision-making research and for applications of traditional signal detection theory are discussed.

Decision Making↗

PubMed2006

Confounder selection in environmental epidemiology: assessment of health effects of prenatal mercury exposure.

PURPOSE: The purpose of the study is to compare different approaches to the identification of confounders needed for analyzing observational data. Whereas standard analysis usually is conducted as if the confounders were known a priori, selection uncertainty also must be taken into account. METHODS: Confounders were selected by using backward elimination (BE), change in estimate (CIE) method, Akaike information criterion, Bayesian information criterion (BIC), and an empirical approach using a priori information. A modified ridge regression estimator, which shrinks effects of confounders toward zero, also was considered. For each criterion, uncertainty in the estimated exposure effect was assessed by using bootstrap simulations for which confounders were selected in each sample. These methods were illustrated by using data for mercury neurotoxicity in Faroe Islands children. Point estimates and standard errors of mercury effects on confounder-sensitive neurobehavioral outcomes were calculated for each selection procedure. RESULTS: The full model and the empirical a priori model showed approximately the same precision, and these methods were (slightly) inferior to only modified ridge regression. Lower precisions were obtained by using BE with a low cutoff level, BIC, and CIE. CONCLUSIONS: Standard analysis ignores model selection uncertainty and is likely to yield overoptimistic inferences. Thus, the traditional BE procedure with p = 5% should be avoided. If data-dependent procedures are required for confounder identification, we recommend that inferences be based on bootstrap statistics to describe the selection process.

Animals↗

PubMed2000

Wildlife population assessment: past developments and future directions.

We review the major developments in wildlife population assessment in the past century. Three major areas are considered: mark-recapture, distance sampling, and harvest models. We speculate on how these fields will develop in the next century. Topics for which we expect to see methodological advances include integration of modeling with Geographic Information Systems, automated survey design algorithms, advances in model-based inference from sample survey data, a common inferential framework for wildlife population assessment methods, improved methods for estimating population trends, the embedding of biological process models into inference, substantially improved models for conservation management, advanced spatiotemporal models of ecosystems, and greater emphasis on incorporating model selection uncertainty into inference. We discuss the kind of developments that might be anticipated in these topics.

Algorithms↗

PubMed2003

The contribution of bubbles to high-frequency sea surface backscatter: a 24-h time series of field measurements.

Measurements of acoustic sea surface backscattering, wind speed, and surface wave spectra were made continually over a 24-h period in an experiment conducted in 26 m of water near the Dry Tortugus collection of islands off south Florida in February 1995. The backscattering measurements were made at a frequency of 30 kHz and a sea surface grazing angle of 20 degrees; a time series of the decibel equivalent of this variable, called SS20, was studied in terms of its dependence on environmental variables. On occasion reliable estimates of scattering in the grazing range 15 degrees-27 degrees were also obtained during the 24 hours. The scattering data exhibited evidence, in terms of scattering level and grazing angle dependence, of scattering from near-surface bubbles rather than scattering from the rough air-sea interface. The scattering data were compared with a model for sigma(b), the apparent backscattering cross section per unit area due to bubble scattering, that is driven by a parameter, beta1, equal to the depth-integrated extinction cross section per unit volume. Using an empirical model for beta1 based on data from a 1977 experiment conducted in pelagic waters, model predictions agreed reasonably well with the 1995 measurements presented here. Additional model-data comparisons were made using four measurements from a 1992 experiment conducted in pelagic waters. Finally, the 24-h time series of acoustic scattering exhibited a hysteresis effect, wherein for a given wind speed, there was a tendency for the scattering level to be higher if prior winds had been falling. A better understanding of this effect is essential to reduce uncertainty in model predictions.

Journal Article↗

PubMed2005

Resources and uncertainties in evaluation of chemicals: basic methodological developments.

Conscious handling of uncertainty is critical in the assessment of chemicals, where the demand of knowledge is high compared to the available amount of resources. Consequently weighing between the use of resources and the acceptance of uncertainty has to be a transparent process and this sets up a challenge in relation to the application of mathematical methods. The maximum entropy principle is a useful paradigm for setting up prior information that is as non-informative as possible. It is shown how the use of partial ordering for deriving ranking results can be considered as a maximum entropy strategy in relation to the model structure uncertainty. In this way it seems possible to solve central parts of the problem of resource optimising risk assessment by combining the precautionary principle and screening before more detailed risk assessment is undertaken. The non-parametric approach of partial order technique further appears to be more transparent than most other ranking methodologies for complex systems. Because in the partial order techniques, no more or less known uncertainty will be hidden in the application of specific weighting factors and functional relationships among the criteria.

Environmental Exposure↗

PubMed1984

Detection and identification of spatial frequency: models and data.

Detection and identification of up to four simple sinusoidal gratings were studied. The experimental results were quantitatively compared to predictions from several models. The models all assumed probabilistically independent channels sensitive to different ranges of spatial frequency. The models differed in the shapes of their underlying distributions and, for detection, their decision rule. Detection and identification of far-apart spatial frequencies were consistent with these models. Thus, uncertainty effects for both detection and identification (the decrease in performance with an increase in the number of possible spatial frequencies) can be explained without assuming that attention capacity is limited.

Attention↗

PubMed2004

Optimum municipal wastewater treatment plant design with consideration of uncertainty.

A newly developed model for the optimum municipal wastewater treatment plant (MWTP) design is presented. Through introducing the interval variables, the model attempts to consider the effects of uncertainties caused by the fluctuation of the wastewater quality and quantity during the design of MWTP. The model solution procedure is illustrated in detail, and a numerical example is given to verify the feasibility and advantage of the model. Furthermore, the possibility of the model application is briefly outlined.

Facility Design and Construction↗

PubMed1998

Measurement and modelling of perceived slant in surfaces represented by freely viewed line drawings.

Simple pictures under everyday viewing conditions evoke impressions of surfaces oriented in depth. These impressions have been studied by measuring the slants of perceived surfaces, with probes (rotating arrowheads) designed to respect the distinctive character of depicted scenes. Converging arguments indicated that the perceived orientation of the probes was near theoretical values. A series of experiments showed that subjects formed well-defined impressions of depicted surface orientation. The literature suggests that perceived objects might be 'flattened', but that was not the general rule. Instead, both mean slant and uncertainty fitted models in which slant estimates are derived in a relatively straightforward way from local relations in the picture. Simplifying pictures tended to make orientation estimates less certain, particularly away from the natural anchor points (vertical and horizontal). The shape of the object affected all aspects of the observed-object/percept relationship. Individual differences were large, and suggest that different individuals used different relationships as a basis for their estimates. Overall, data suggest that everyday picture perception is strongly selective and weakly integrative. In particular, depicted slant is estimated by finding a picture feature which will be strongly related to it if the object contains a particular regularity, not by additive integration of evidence from multiple directly and indirectly relevant sources.

Computer Graphics↗

PubMed1997

Biosphere model for high level waste repository.

Method for environmental impact modeling involving uncertainty, overcoming the disadvantage of providing only one upper bound based on accumulated effects from all extreme events. This method provides a suite of upper and lower bounds based on any subset of such extreme events, to be chosen among by the decision maker.

Ecosystem↗

PubMed2005

Effect of image uncertainty on the dosimetry of trigeminal neuralgia irradiation.

OBJECTIVE: Our objective was to quantify the uncertainty in localization of the trigeminal nerve (TGN) with magnetic resonance imaging (MRI) and computed tomography (CT) and to determine the effect of this uncertainty on gamma-knife dose delivery. METHODS: An MR/CT test phantom with 9, 0.6-mm diameter, copper rings was devised. The absolute ring positions in stereotactic space were determined by the angiographic module of the LGP software. The standard deviation, sigma, in the difference between the absolute and MR-measured or CT-measured coordinates of the rings was determined. The trigeminal nerve in 52 previously treated patients was contoured and expanded by 1sigma and 2sigma margins to model the uncertainty in the location of the nerve. For gamma-knife treatment, a single isocenter was used and was located at the distal cisternal portion of the trigeminal nerve root. Irradiation methods included a 4-mm collimator, 90 Gy to isocenter and a 4&8-mm collimator, 70 Gy to isocenter. A patient outcome survey that sampled pain relief and morbidity was done. RESULTS: The MR coordinate sigma was 0.7 mm left-right, 0.8 mm anterior-posterior, and 0.6 mm superior-inferior, and the CT coordinate sigma was 0.4 mm left-right, 0.2 mm anterior-posterior, and 0.2 mm superior-inferior. A 45% higher dose line covered the TGN with the 4&8-mm method. No significant increase in pain reduction or morbidity occurred. CONCLUSIONS: The uncertainty of target location by MRI is more than twice that found in CT imaging. The 4&8-mm collimator method covers the trigeminal root cross section with a higher isodose line than does the 4-mm method. This higher dose did not significantly reduce pain or increase morbidity.

Humans↗

PubMed1987

Kinetic data analysis with a noisy input function.

Methods of parameter estimation are proposed for the analysis of dynamic experiments in which the input function is noisy. Noise in the input function leads to uncertainties in the calculated model-predicted values, and therefore the covariance matrix of the residuals is a function of the model parameters. These statistical uncertainties in the model-predicted values significantly change the nature of the fitting process and the quality of the results. The proposed optimisation methods use weighted least-squares criteria, and three choices for the weighting matrix are considered. The proposed weighting matrices, in order of complexity are: (1) the identity matrix (no weighting), (2) the covariance matrix of the data (ignoring the noise in the input function), and (3) the full covariance matrix of the residuals (incorporating both the noise in the data and the noise in the input function). The methodology is applied to dynamic emission tomography studies of the heart, where the blood (input) and tissue tracer concentrations at each time are derived from two regions of interest in the same tomographic slice. Computer stimulations of compartmental systems show that parameters and their covariance matrix are more accurately estimated when the full covariance matrix of the residuals is used as a weighting matrix rather than either of the other two methods. For the practical example considered, parameter bias was increased by a factor of at least four when the noise in the input function was ignored, and one parameter had a bias of 24% when the unweighted least-squares criterion was used.

Computer Simulation↗

PubMed2000

Analysis of data from spilling experiments performed with liquid hydrogen.

This work describes the modelling of liquid hydrogen release experiments using the ADREA-HF 3-D time dependent finite volume code for cloud dispersion, jointly developed by DEMOKRITOS and JRC-Ispra. The experiments were performed by Batelle Ingenieurtechnik for BAM (Bundesanstalt fur Materialforschung und Prufung), Berlin, in the frame of the Euro-Quebec-Hydro-Hydrogen-Pilot-Project and they mainly deal with LH2 near ground releases between buildings. In the present study, the experimental trial #5 was assumed for simulation due to the fact that in this release the largest number of sensor readings were obtained. The simulations illustrated the complex behaviour of LH2 dispersion in presence of buildings, characterized by complicated wind patterns, plume back flow near the source, dense gas behaviour at near range and significant buoyant behaviour at the far range. The simulations showed the strong effect of ground heating in the LH2 dispersion. The model also revealed major features of the dispersion that had to do with the "dense" behaviour of the cold hydrogen and the buoyant behaviour of the "warming-up" gas as well as the interaction of the building and the release wake. Such a behaviour was in qualitative and even quantitative agreement with the experiment. The results are given in terms of concentration time series, scatter plots, contour plots, wind field vector plots and 3-D concentration wireframes. Given all experiment uncertainties, the model gives reasonable results on concentrations levels.

Accidents↗